[PATCH v2 04/11] perf/uprobe: RCU-ify find_uprobe()

[Peter Zijlstra <peterz@infradead.org>]

With handle_swbp() triggering concurrently on (all) CPUs, tree_lock
becomes a bottleneck. Avoid treelock by doing RCU lookups of the
uprobe.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/events/uprobes.c |   49 +++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 40 insertions(+), 9 deletions(-)

--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -40,6 +40,7 @@ static struct rb_root uprobes_tree = RB_
 #define no_uprobe_events()	RB_EMPTY_ROOT(&uprobes_tree)
 
 static DEFINE_RWLOCK(uprobes_treelock);	/* serialize rbtree access */
+static seqcount_rwlock_t uprobes_seqcount = SEQCNT_RWLOCK_ZERO(uprobes_seqcount, &uprobes_treelock);
 
 #define UPROBES_HASH_SZ	13
 /* serialize uprobe->pending_list */
@@ -54,6 +55,7 @@ DEFINE_STATIC_PERCPU_RWSEM(dup_mmap_sem)
 struct uprobe {
 	struct rb_node		rb_node;	/* node in the rb tree */
 	refcount_t		ref;
+	struct rcu_head		rcu;
 	struct rw_semaphore	register_rwsem;
 	struct rw_semaphore	consumer_rwsem;
 	struct list_head	pending_list;
@@ -587,12 +589,25 @@ set_orig_insn(struct arch_uprobe *auprob
 			*(uprobe_opcode_t *)&auprobe->insn);
 }
 
+static struct uprobe *try_get_uprobe(struct uprobe *uprobe)
+{
+	if (refcount_inc_not_zero(&uprobe->ref))
+		return uprobe;
+	return NULL;
+}
+
 static struct uprobe *get_uprobe(struct uprobe *uprobe)
 {
 	refcount_inc(&uprobe->ref);
 	return uprobe;
 }
 
+static void uprobe_free_rcu(struct rcu_head *rcu)
+{
+	struct uprobe *uprobe = container_of(rcu, struct uprobe, rcu);
+	kfree(uprobe);
+}
+
 static void put_uprobe(struct uprobe *uprobe)
 {
 	if (refcount_dec_and_test(&uprobe->ref)) {
@@ -604,7 +619,7 @@ static void put_uprobe(struct uprobe *up
 		mutex_lock(&delayed_uprobe_lock);
 		delayed_uprobe_remove(uprobe, NULL);
 		mutex_unlock(&delayed_uprobe_lock);
-		kfree(uprobe);
+		call_rcu(&uprobe->rcu, uprobe_free_rcu);
 	}
 }
 
@@ -653,10 +668,10 @@ static struct uprobe *__find_uprobe(stru
 		.inode = inode,
 		.offset = offset,
 	};
-	struct rb_node *node = rb_find(&key, &uprobes_tree, __uprobe_cmp_key);
+	struct rb_node *node = rb_find_rcu(&key, &uprobes_tree, __uprobe_cmp_key);
 
 	if (node)
-		return get_uprobe(__node_2_uprobe(node));
+		return try_get_uprobe(__node_2_uprobe(node));
 
 	return NULL;
 }
@@ -667,20 +682,32 @@ static struct uprobe *__find_uprobe(stru
  */
 static struct uprobe *find_uprobe(struct inode *inode, loff_t offset)
 {
-	struct uprobe *uprobe;
+	unsigned int seq;
 
-	read_lock(&uprobes_treelock);
-	uprobe = __find_uprobe(inode, offset);
-	read_unlock(&uprobes_treelock);
+	guard(rcu)();
 
-	return uprobe;
+	do {
+		seq = read_seqcount_begin(&uprobes_seqcount);
+		struct uprobe *uprobe = __find_uprobe(inode, offset);
+		if (uprobe) {
+			/*
+			 * Lockless RB-tree lookups are prone to false-negatives.
+			 * If they find something, it's good. If they do not find,
+			 * it needs to be validated.
+			 */
+			return uprobe;
+		}
+	} while (read_seqcount_retry(&uprobes_seqcount, seq));
+
+	/* Really didn't find anything. */
+	return NULL;
 }
 
 static struct uprobe *__insert_uprobe(struct uprobe *uprobe)
 {
 	struct rb_node *node;
 
-	node = rb_find_add(&uprobe->rb_node, &uprobes_tree, __uprobe_cmp);
+	node = rb_find_add_rcu(&uprobe->rb_node, &uprobes_tree, __uprobe_cmp);
 	if (node)
 		return get_uprobe(__node_2_uprobe(node));
 
@@ -702,7 +729,9 @@ static struct uprobe *insert_uprobe(stru
 	struct uprobe *u;
 
 	write_lock(&uprobes_treelock);
+	write_seqcount_begin(&uprobes_seqcount);
 	u = __insert_uprobe(uprobe);
+	write_seqcount_end(&uprobes_seqcount);
 	write_unlock(&uprobes_treelock);
 
 	return u;
@@ -936,7 +965,9 @@ static void delete_uprobe(struct uprobe
 		return;
 
 	write_lock(&uprobes_treelock);
+	write_seqcount_begin(&uprobes_seqcount);
 	rb_erase(&uprobe->rb_node, &uprobes_tree);
+	write_seqcount_end(&uprobes_seqcount);
 	write_unlock(&uprobes_treelock);
 	RB_CLEAR_NODE(&uprobe->rb_node); /* for uprobe_is_active() */
 	put_uprobe(uprobe);